This invention relates to an ATM switch device and an ATM method for use in switching ATM cells in an ATM network.
In general, a conventional ATM switch device of the type described (includes an input port portion and an output port portion) is operable to receive an ATM cell as an input cell at an input port and to deliver the input cell to an output port as an output cell after the input cell is stored in the ATM switch device. Herein, it is to be noted that such an ATM cell has a fixed length of fifty-three bytes in total and is structured by a header of five bytes and a payload of forty-eight bytes. The header has indentifiers, such as VPI, VCI, and the like while the payload has a sequence of data signals, and so on.
In such an ATM network which uses an ATM cell of a fixed length, after an ATM cell is formed once, all cells have the same structure. Therefore, it is advantageous that the ATM network never recognizes a structure of an original information structure. Under the circumstances, it has been expected that the ATM network is suitable for multimedia communications.
In a method of transmitting an ATM cell through an ATM switch device, there are a wide variety of services, such as a constant bit rate (CBR) service, a variable bit rate (VBR) service, an available bit rate (ABR) service, an unspecified bit rate (UBR) service, and the like. Herein, it is to be noted that the CBR service is for transmitting a moving picture signal and an audio signal at a constant transmission rate while the VBR service is for varying a transmission rate of a moving picture signal and an audio signal at a real time. On the other hand, the ABR service is for varying a transmission rate in consideration of traffic congestion of an ATM network while the UBR method is for carrying out transmission without specifying a transmission rate.
In considering a variety of the methods, it is preferable that the ATM switch device is matched with all of the above-mentioned methods. In other words, the ATM switch device can desirably distinguish all of the methods from one another as service classes to carry out control operation which corresponds to each of the service classes.
Moreover, it should be considered that the ATM cells which is supplied to and transmitted from the ATM switch device are classified into a single cast cell which is sent to a single output port and an a multicast cell which is simultaneously delivered to a plurality of output ports. Accordingly, it is preferable that the ATM switch device can not only process the single cast cell but also can process the multicast cell.
Herein, proposal has been made about a wide variety of ATM switch devices which are divided into three groups. One of the groups is of a type which includes a plurality of input ports and a plurality of input buffers arranged for the respective input ports to store the input cells while another one of the groups is of a type which includes a plurality of output ports arranged for the respective output ports to store the output cells. The last one of the groups is of a type which includes a shared buffer shared by a plurality of output ports and which will be referred to as a shared buffer type hereinafter.
Among others, a recent interest has been directed to the ATM switch device of the shared buffer type because it is excellent in comparison with the other groups of the ATM switch devices in view of the fact that the shared buffer can reduce a memory capacity.
At any rate, all the ATM switch devices have a common problem that the ATM cells are discarded when traffic congestion takes place at a certain output port, which causes an overflow to occur in the input, the output, and the shared buffers.
This problem will be mentioned about the shared buffer type in detail hereinunder. Such an ATM switch device of the shared buffer type is disclosed, for example, in Japanese Patent Unexamined Publication No. Hei 5-153,150, namely, 153,150/1993 (will be referred to as Reference). The ATM switch device disclosed therein includes a shared buffer shared by a plurality of output ports to store each cell and an address administration buffer arranged for each of the output port to store an address of the shared buffer assigned to each cell. In addition, an idle address buffer is also included to store an idle address of the shared buffer.
Furthermore, description is made in Reference about a conventional ATM switch device which includes a threshold value monitoring unit. The threshold value monitoring unit monitors whether or not an activity ratio of each address administration buffer exceeds a threshold value set therein. In addition, the threshold value monitoring unit produces a cell discard indication when the the activity ratio exceeds the threshold value. The cell discard indication is sent to a cell discard unit and, as a result, the cell in question is discarded.
From this fact, it is readily understood that selective discard is made about the cell which is directed to the output port corresponding to one of the address administration buffers that exceeds the threshold value and that the threshold values are determined for the respective output ports. With this structure, the cells are discarded even when the conventional ATM switch device as a whole affords to receive and transmit the cells. Therefore, discarding the cells comparatively frequently happens in the conventional ATM switch device.
In order to reduce cell discard as small as possible, Reference proposes a method of monitoring a total amount of cells stored in the shared buffer shared by the output ports. In this method, when a load begins to be concentrated on a specific one of the output ports with the total amount of the stored cells exceeding a predetermined value, an input regulation or restriction is imposed on the shared buffer.
In other words, when the total amount of the stored cells exceeds the predetermined value, the input regulation is made in this method about the cells which are maximum in number and which are delivered to the specific output port. As a result, the cells delivered to the specific output port are discarded during the input regulation or restriction even when the cells are to be quickly processed in a real time.
As mentioned above, this method detects the specific output port to which a maximum number of cells is delivered. In addition, the cells to be delivered to the specific output port are subjected to regulation or restriction and are consequently discarded as long as the total amount of the stored cells exceeds the predetermined value. Such regulation or restriction state is continued in connection with the specific output port until the total amount of the stored cells becomes equal to or smaller than the predetermined value.
This brings about inconvenience that the cell regulation of the specific output port is not released or stopped when the total amount of the cells is kept over the predetermined value by an increase of the cells directed to the output ports other than the specific output port in spite of a decrease of the cells directed to the specific output port.
Moreover, no consideration is made at all in Reference about input regulation to be carried out in the cases where the different service classes are present and where the multicast cell is present together with the single cast cell. In addition, no teaching is also made about the case where each output port has a different concentration of the cells to be allowed.